function YYYYMMDDstart()
{
    MonHead = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

    //先给年下拉框赋内容
    var y  =  new Date().getFullYear();
    document.reg_testdate.YYYY.options.length = 0; //去除空白选项
    for (var i = 1900; i <= y; i++) //以今年为准，前100年
        document.reg_testdate.YYYY.options.add(new Option(" "+ i, i));

    //赋月份的下拉框
    document.reg_testdate.MM.options.length = 0; //去除空白选项
    for (var i = 1; i < 13; i++)
        document.reg_testdate.MM.options.add(new Option(" " + i + " 月", i));

    document.reg_testdate.YYYY.value = y;
    document.reg_testdate.MM.value = new Date().getMonth() + 1;
    var n = MonHead[new Date().getMonth()];
    if (new Date().getMonth() ==1 && IsPinYear(document.reg_testdate.YYYY.value))
        n++;
    writeDay(n); //赋日期下拉框
    document.reg_testdate.DD.value = new Date().getDate();
}

window.addEventListener('load', YYYYMMDDstart, false);

function YYYYDD(str) //年发生变化时日期发生变化(主要是判断闰平年)
{
    var MMvalue = document.reg_testdate.MM.options[document.reg_testdate.MM.selectedIndex].value;
    if (MMvalue == "")
    {
        var e = document.reg_testdate.DD;
        optionsClear(e);
        return;
    }
    var n = MonHead[MMvalue - 1];
    if (MMvalue ==2 && IsPinYear(str))
        n++;
    writeDay(n)
}

function MMDD(str)   //月发生变化时日期联动
{
    var YYYYvalue = document.reg_testdate.YYYY.options[document.reg_testdate.YYYY.selectedIndex].value;
    if (YYYYvalue == "")
    {
        var e = document.reg_testdate.DD;
        optionsClear(e);
        return;
    }
    var n = MonHead[str - 1];
    if (str == 2 && IsPinYear(YYYYvalue))
        n++; //闰年29天
    writeDay(n)
}

function writeDay(n)   //据条件写日期的下拉框
{
    var d = document.reg_testdate.DD;
    optionsClear(d);
    for (var i=1; i<(n+1); i++)
        d.options.add(new Option(" "+ i, i));
}

function IsPinYear(year)//判断是否闰平年
{
    return(0 == year%4 && (year%100 !=0 || year%400 == 0));
}

function optionsClear(e)
{
    e.options.length = 0;
}